1. Field of the Invention
The present invention relates to a method and a mechanism for feeding sheets, one by one, from a sheet stack to a predetermined location.
2. Description of the Related Art
Sheet feeders are employed to feed stacked sheets, e.g., photosensitive sheets such as photographic films, to an exposure position or to feed sheets with images recorded thereon to an image developing position.
Such a sheet feeder usually comprises a plurality of suction cups coupled to a vacuum generator. The suction cups are pressed against an uppermost one of stacked sheets, attract the sheet under a vacuum developed in the suction cups, remove the sheet from the sheet stack, and deliver the sheet to a sheet delivery mechanism.
When the uppermost sheet is drawn by the suction cups, one or more sheets beneath the uppermost sheet tend to stick to the uppermost sheet, and to be removed simultaneously from the sheet stack. To prevent a plurality of sheets from being fed at the same time, the suction cups as they attract the uppermost sheet are angularly and/or linearly moved to give a swinging motion to the sheet.
However, the sheet feeder capable of giving a swinging motion to the sheet while it is being fed by the suction cups is relatively complex in structure, and highly costly to manufacture. Since the sheet attracted by the suction cups is moved along an intricate path for swinging movement, it takes a relatively long period of time to feed the sheet from the sheet stack, failing to feed the sheets efficiently.
The sheet delivery mechanism has a pair of feed rollers held in rolling contact with each other. After the leading end of a sheet attracted by the suction cups is inserted between the feed rollers, the feed rollers are rotated to deliver the sheet to a predetermined position, e.g., toward an automatic image developing apparatus.
The suction cups of the sheet feeder are not capable of attracting the leading end of the sheet. Therefore, when the sheet is attracted by the suction cups, the leading end thereof is liable to flex downwardly due to gravity. Inserting the leading end of the attracted sheet between the feed rollers requires that the position of the suction cups be controlled with high accuracy. Thus, the process of controlling the position of the suction cups becomes considerably complex. Another problem is that if the leading end of the sheet attracted by the suction cups is curled, it cannot smoothly be inserted between the feed rollers.
One solution has been to employ a sheet delivery. mechanism comprising a feed roller and a swingable nip roller. This sheet delivery mechanism operates as follows: After the suction cups which attract a sheet have moved beyond the feed roller, the nip roller is angularly displaced from the trailing end of the sheet to nip the sheet between the feed roller and the nip roller, and the feed roller and the nip roller cooperate with each other to feed the sheet.
The sheet delivery mechanism, however, needs a dedicated actuator for swinging the nip roller in addition to an actuator for moving the suction cups. Consequently, the overall sheet feed assembly is highly complex and expensive. Furthermore, inasmuch as it is necessary to move the suction cups beyond the feed roller to a position forwardly of the feed roller, the distance that is traversed by the suction cups is relatively large. The relatively large distance traversed by the suction cups results in a reduction in the efficiency with which the sheets are fed one by one.